Isoflavones contained in soybean (for example, soybean isoflavones such as daidzein, genistein, and glycitein) have structures similar to estradiol, and an anti-estrogenic action and estrogen-like action associated with binding to an estrogen receptor (hereinafter referred to as ER). Epidemiologic studies and interventional studies on soybean isoflavones conducted thus far suggest that soybean isoflavones have a preventive effect, attributable to the anti-estrogenic action, on hormone-dependent cancers such as breast cancer and prostate cancer; and an improvement effect, attributable to the estrogen-like action, on menopausal disorders, postmenopausal osteoporosis and hyperlipidaemia.
Recently, it has been reported that the active principle of the physiological effects of these soybean isoflavones may be equol, a metabolite of daidzein. More specifically, it has been reported that equol has a greater ability to bind to ER (especially to ERβ) than soybean isoflavones, and that it has a remarkably high transition capability to target organs such as breast and prostate tissues (refer to Non-Patent Documents 1-4). Moreover, a case-control study reports that there are significantly few breast cancer and prostate cancer patients who can produce equol. The bone density- and lipid metabolism-improvement effects of soybean isoflavones were examined in postmenopausal women categorized into two groups: those who could produce equol, and those who could not. A significant improvement in those who could produce equol was observed.
Equol is produced by the metabolism of daidzein by enteric bacteria. The ability to produce equol varies between individuals, and the percentage of Japanese who can produce equol production is reportedly about 50%. That is, about 50% of Japanese are unable to produce equol (equol non-producers). Such individuals cannot enjoy the useful physiological benefits provided by the action of equol, even if they ingest soybeans and processed soybean foods. Therefore, in order for an equol non-producer to attain the useful physiological benefits provided by the action of equol, ingesting equol itself is thought to be effective.
Equol-producing microorganisms are already known; for example, Bacteroides E-23-15 (FERM BP-6435), Streptococcus E-23-17 (FERM BP-6436), Streptococcus A6G225 (FERM BP-6437) and Lactococcus 20-92 (FERM BP-10036) have been isolated from within the human intestine by the present inventors (refer to Patent Documents 1 and 2).
Consequently, it is thought that if a fermented product containing equol-producing microorganisms in the state of living cells could be provided, ingestion of an equol-producing microorganism could become possible, enabling individuals to benefit from the useful effects of the equol-producing microorganisms. However, if a fermented product is produced using an equol-producing microorganism according to common methods, the equol-production ability of the microorganism is problematically lost; and therefore, a fermented product containing equol-producing microorganisms in the state of living cells cannot be obtained. In addition, since equol-producing microorganisms have a strong tendency to lose their equol-production ability due to low pH conditions or aerobic storage, even if a fermented product could be produced while maintaining the equol-production ability of the microorganism, the equol-production ability, not being able to endure storage, would be lost at the distribution stage.
With such prior art as the background, the development of a fermented product containing microorganisms in the state of living cells by which the equol-production ability can be maintained is desired.